Characterization and physical-chemistry of methoxypoly(ethylene glycol)-g-chitosan

Methoxypoly(ethyleneglycol)-graft-chitosan (PEG-g-Ch) was prepared by grafting polyethyleneglycol into chitosans (Ch) exhibiting different average degree of deacetylation (60% < DD¯ < 95%). ¹H NMR showed that PEG-g-Ch derivatives presented high average degree of N-substitution (DS¯ ≈ 40%) and such derivatives exhibited full water solubility at 1.0 < pH < 11.0. The mPEG-g-Ch derivatives displayed much lower intrinsic viscosity (20 mL g^-1 < [η] < 110 mL g^-1) as compared to the parent chitosans (440 mL g^-1 < [η] < 1650 mL g^-1) due to extensive exposition of PEG chains to the aqueous medium and compact coiling of the chitosan backbone. The presence of numerous PEG chains grafted into chitosan also determined the crystalline arrangement and the thermal stability of PEG-g-Ch derivatives. The rheological study showed that the concentrated aqueous solutions of PEG-g-Ch derivatives displayed pseudoplastic behavior regardless of the parent chitosans´ characteristics and no dependence of dynamic viscosity on the temperature. However, PChD2 (DD¯ ≈ 76%; [η] ≈ 1201 mL g^-1) showed a distinct rheological behavior as it formed a physically cross-linked hydrogel that exhibited a thermo-induced sol-gel transition at ≈38 °C.